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Aerodynamics acording to Myth Busters!



 
 
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  #1  
Old July 27th 07, 01:25 PM posted to rec.aviation.homebuilt
Blueskies
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Posts: 979
Default Aerodynamics acording to Myth Busters!


"Denny" wrote in message ups.com...

Paper material in the bed of my truck at 55mph rotates once or trwice
in that standing vortex then vanishes over the tailgate - never to be
seen again...
Any empty, plastic gas can left against the tailgate slides up against
the back of the cab - again that standing vortex...
Now, that the gate up has less drag overal still amazes me, in spite
of knowing what is theoretically happening...

denny


I forget which brand truck we were looking at (Ford?), but it had a sculpted shape to the top of the tailgate about 8"
wide. The salesman said it helped to improve gas mileage...


  #2  
Old July 27th 07, 01:49 PM posted to rec.aviation.homebuilt
Morgans[_2_]
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Posts: 3,924
Default Aerodynamics acording to Myth Busters!


"Blueskies" wrote

I forget which brand truck we were looking at (Ford?), but it had a sculpted
shape to the top of the tailgate about 8" wide. The salesman said it helped to
improve gas mileage...


He was wrong. It is to provide extra clearance for people towing 5th wheel or
gooseneck trailers.
--
Jim in NC

  #3  
Old July 27th 07, 02:00 PM posted to rec.aviation.homebuilt
Blueskies
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Posts: 979
Default Aerodynamics acording to Myth Busters!


"Morgans" wrote in message ...

"Blueskies" wrote

I forget which brand truck we were looking at (Ford?), but it had a sculpted shape to the top of the tailgate about
8" wide. The salesman said it helped to improve gas mileage...


He was wrong. It is to provide extra clearance for people towing 5th wheel or gooseneck trailers.
--
Jim in NC


No, it was not a 5th wheel clearance thing. I did not describe it very clearly. Regular rectangular tailgate with a wide
upper (top when closed) edge...


  #4  
Old July 27th 07, 02:28 PM posted to rec.aviation.homebuilt
Ernest Christley
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Posts: 199
Default Aerodynamics acording to Myth Busters!

Blueskies wrote:
"Morgans" wrote in message ...
"Blueskies" wrote
I forget which brand truck we were looking at (Ford?), but it had a sculpted shape to the top of the tailgate about
8" wide. The salesman said it helped to improve gas mileage...

He was wrong. It is to provide extra clearance for people towing 5th wheel or gooseneck trailers.
--
Jim in NC


No, it was not a 5th wheel clearance thing. I did not describe it very clearly. Regular rectangular tailgate with a wide
upper (top when closed) edge...



You see it on a lot of trucks. It used to be that a box van was just
that, a box. A decade or so ago, they started rounding the front edges
of the box. Can you say "fairing". Some started rounding the back
corners also. I've seen SUV's with what looks like reverse scoops that
could have to catch the air passing over the top of the vehicle and blow
it down the back. Counterintuitive until you realize that attached flow
offers less drag.

The thick tailgate with the rounded edge keeps the air in attached flow
longer. Even a small amount, just a few inches, offers major
improvements over the previous generations 90 degree chop offs.

  #5  
Old July 27th 07, 03:22 PM posted to rec.aviation.homebuilt
Peter Dohm
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Posts: 1,754
Default Aerodynamics acording to Myth Busters!


You see it on a lot of trucks. It used to be that a box van was just
that, a box. A decade or so ago, they started rounding the front edges
of the box. Can you say "fairing". Some started rounding the back
corners also. I've seen SUV's with what looks like reverse scoops that
could have to catch the air passing over the top of the vehicle and blow
it down the back. Counterintuitive until you realize that attached flow
offers less drag.

You have been tricked! The attached downward flow has nothing to do with
drag; but is to keep exhaust and also road dirt away from the rear of the
vehicle. Similar ducts became a standard feature of tull sized station
wagons during the '60s and '70s.

The thick tailgate with the rounded edge keeps the air in attached flow
longer. Even a small amount, just a few inches, offers major
improvements over the previous generations 90 degree chop offs.

I have yet to examine the rounded tail gate, but have my doubts. Suffice it
to say that some salesmen are even more creative than the brochure
writers--so you might compare the company litterature to the salesman's
assertion.

Peter


  #6  
Old July 28th 07, 02:13 AM posted to rec.aviation.homebuilt
Morgans[_2_]
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Posts: 3,924
Default Aerodynamics acording to Myth Busters!


"Ernest Christley" wrote

I've seen SUV's with what looks like reverse scoops that could have to catch
the air passing over the top of the vehicle and blow it down the back.
Counterintuitive until you realize that attached flow offers less drag.


That scoop helps drag, but more importantly, it prevents carbon monoxide from
entering in the back window when it is open and driving at highway speeds.

When a chopped off SUV has the window open, that low pressure area in the back
gathers up the exhaust, and the swirling motion of the vortex allows some to
enter in though the open window. Not good. The scoop supplies fresh air
flowing down from the top, and keeps the bad air down lower than the open
window.

They have been doing that since the 60's, on old station wagons, before they
cared about mileage and aerodynamics.
--
Jim in NC

  #7  
Old July 28th 07, 03:25 AM posted to rec.aviation.homebuilt
Bill Daniels
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Posts: 687
Default Aerodynamics acording to Myth Busters!

You guys haven't mentioned the biggie - roof racks. Removing the cross bars
from my Grand Cherokee added 2MPG. Glider pilots look at roof racks and see
open air brakes. I can't believe the number of vehicles I see in Denver
with ski racks in July.

Then, of course, you can park the truck. I understand that trucks have very
low drag when parked.

If the vehicle has a MPG computer, you can see what speed does to MPG. At
65, my Jeep gets 19MPG. At 55, it gets 24MPG. At 45, it gets 36MPG. If
you do the numbers, saving two minutes by driving fast costs big time.
Those are REALLY expensive minutes - more than the rental on some airplanes.

Bill Daniels


"Morgans" wrote in message
...

"Ernest Christley" wrote

I've seen SUV's with what looks like reverse scoops that could have to
catch the air passing over the top of the vehicle and blow it down the
back. Counterintuitive until you realize that attached flow offers less
drag.


That scoop helps drag, but more importantly, it prevents carbon monoxide
from entering in the back window when it is open and driving at highway
speeds.

When a chopped off SUV has the window open, that low pressure area in the
back gathers up the exhaust, and the swirling motion of the vortex allows
some to enter in though the open window. Not good. The scoop supplies
fresh air flowing down from the top, and keeps the bad air down lower than
the open window.

They have been doing that since the 60's, on old station wagons, before
they cared about mileage and aerodynamics.
--
Jim in NC



  #8  
Old July 28th 07, 03:49 AM posted to rec.aviation.homebuilt
Alan Baker
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Posts: 244
Default Aerodynamics acording to Myth Busters!

In article ,
"Bill Daniels" bildan@comcast-dot-net wrote:

You guys haven't mentioned the biggie - roof racks. Removing the cross bars
from my Grand Cherokee added 2MPG. Glider pilots look at roof racks and see
open air brakes. I can't believe the number of vehicles I see in Denver
with ski racks in July.

Then, of course, you can park the truck. I understand that trucks have very
low drag when parked.

If the vehicle has a MPG computer, you can see what speed does to MPG. At
65, my Jeep gets 19MPG. At 55, it gets 24MPG. At 45, it gets 36MPG. If
you do the numbers, saving two minutes by driving fast costs big time.
Those are REALLY expensive minutes - more than the rental on some airplanes.



Real world case; let's use your figures for simplicity's sake:

I want to drive from Vancouver to Kelowna. A little check with Google
Maps tells us that that's a distance of...

....393 kilometers, or about 245 miles.

At 65, that takes me about 3 hours, 45 minutes and I use about 13
gallons of gas.

At 55, it takes 4 hours, 30 minutes and I use about 10 gallons of gas.

Even at Canadian gas prices, those 10 litres of fuel cost me only about
$11.50. Is that really too much to pay to save 45 minutes of my precious
time? 25 cents a minute seem pretty cheap to me.




Bill Daniels


"Morgans" wrote in message
...

"Ernest Christley" wrote

I've seen SUV's with what looks like reverse scoops that could have to
catch the air passing over the top of the vehicle and blow it down the
back. Counterintuitive until you realize that attached flow offers less
drag.


That scoop helps drag, but more importantly, it prevents carbon monoxide
from entering in the back window when it is open and driving at highway
speeds.

When a chopped off SUV has the window open, that low pressure area in the
back gathers up the exhaust, and the swirling motion of the vortex allows
some to enter in though the open window. Not good. The scoop supplies
fresh air flowing down from the top, and keeps the bad air down lower than
the open window.

They have been doing that since the 60's, on old station wagons, before
they cared about mileage and aerodynamics.
--
Jim in NC


--
Alan Baker
Vancouver, British Columbia
"If you raise the ceiling four feet, move the fireplace from that wall
to that wall, you'll still only get the full stereophonic effect if you
sit in the bottom of that cupboard."
  #9  
Old July 28th 07, 04:40 AM posted to rec.aviation.homebuilt
cavelamb himself[_2_]
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Posts: 23
Default Aerodynamics acording to Myth Busters!

Bill Daniels wrote:
You guys haven't mentioned the biggie - roof racks. Removing the cross bars
from my Grand Cherokee added 2MPG. Glider pilots look at roof racks and see
open air brakes. I can't believe the number of vehicles I see in Denver
with ski racks in July.

Then, of course, you can park the truck. I understand that trucks have very
low drag when parked.

If the vehicle has a MPG computer, you can see what speed does to MPG. At
65, my Jeep gets 19MPG. At 55, it gets 24MPG. At 45, it gets 36MPG. If
you do the numbers, saving two minutes by driving fast costs big time.
Those are REALLY expensive minutes - more than the rental on some airplanes.

Bill Daniels


Can't argue against the open drag brake idea.

I have a nice pair on my Blazer.
My impression is that they are there to enhance roll-over protection.

I don't have the MPG computer, but I just drove down to Centerville
last weekend. 120 miles alnost exactly.

V6 4 speed automatic - with highay gearing, it's NOT a tow truck.
(and when I hook the boat on the back the Blazer whines and whimpers)

22 MPG going down at 75-85 mph.
26 coming back ar 65.

But about 18 around town...

Bringing the boat home from Khema - in stop and go Houston traffic -
climbing the ovepasses from a standing stop -
first 100 miles at 10 to 25 MPH took 10 gallons.

After that, at about 50 MPH I got more like 15-16 MPG.

(The boat is 18 ft long, but over 1500 pounds - plus a heavy trailer)

As they say, YMMV.

Sometimes a LOT...

Richard
  #10  
Old July 29th 07, 03:04 PM posted to rec.aviation.homebuilt
Peter Dohm
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Posts: 1,754
Default Aerodynamics acording to Myth Busters!


"Bill Daniels" bildan@comcast-dot-net wrote in message
. ..
You guys haven't mentioned the biggie - roof racks. Removing the cross

bars
from my Grand Cherokee added 2MPG. Glider pilots look at roof racks and

see
open air brakes. I can't believe the number of vehicles I see in Denver
with ski racks in July.

Then, of course, you can park the truck. I understand that trucks have

very
low drag when parked.

If the vehicle has a MPG computer, you can see what speed does to MPG. At
65, my Jeep gets 19MPG. At 55, it gets 24MPG. At 45, it gets 36MPG. If
you do the numbers, saving two minutes by driving fast costs big time.
Those are REALLY expensive minutes - more than the rental on some

airplanes.

Bill Daniels


Those numbers really bother me. The numbers are all fairly low; but the
curve shape fits aerodynamic drag, with no other influence such as tires or
"pumping losses." In effect, the numbers fit a much larger, but
aerodynamically atrocious and very lightly loaded vehicle. Therefore, I
really doubt that your mileage computer is telling you the whole truth.

Have you 1) verified you odometer against at least 10 miles of highway mile
markers, 2) verified your speedometer reading at 60MPH as a result of the
same test, and 3) checked the cumulative fuel mileage, as shown on the
computer, against your fuel mileage calculated in the usual way?

Peter



 




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